Back-up bilge pumps are always a good idea, however there are a few points to consider during installation to ensure they work properly when needed.  Here the back-up pump float switch is located a suitable height above the primary pump switch, however the back-up pump itself is located at the same level as the primary pump.  Back-up pumps should ideally be mounted and configured to turn on when bilge water level reaches around 4 to 6 inches above the cut on point for the primary pump.  This prevents the back-up pump from resting in the normal accumulation of bilge water, where it can become clogged with sludge and debris or seized from disuse.